1. Field of the Invention
The present invention relates to a solid-state imaging apparatus, a driving method of the same and an imaging system for use in a scanner, a video camera, a digital still camera and the like.
2. Related Background Art
In a conventional solid-state imaging apparatus, there has been generally known a pixel configured such that a photoelectric conversion element performing photoelectric conversion on incident light also serves as an accumulating element for accumulating an electric charge.
In contrast to this, Japanese Patent Application Laid-Open No. 2004-111590 (hereinafter referred to simply as Patent Document 1) discloses a technique for providing the accumulating element separately from the photoelectric conversion element. According to Patent Document 1, a global shutter can be implemented by transferring the electric charge accumulated in the photoelectric conversion element from the photoelectric conversion element to the accumulating element for all the pixels at the same time.
Alternatively, Japanese Patent Application Laid-Open No. 2006-246450 (hereinafter referred to simply as Patent Document 2) discloses a configuration in which the accumulating element is provided separately from the photoelectric conversion element and most of the electric charges generated by the photoelectric conversion element is not accumulated in the photoelectric conversion element but is transferred to an electric charge accumulating region. FIGS. 9A to 9G are reproduced from FIG. 6 of Patent Document 2. An N-type photodiode layer 301 in FIGS. 9A to 9G forms a PN junction with a P-type silicon substrate. A light guide 302 located above the photodiode has a higher dielectric constant than that of its surrounding region and collects incident light. A surface micro-lens 303 guides light entered through the surface to the light guide 302. An N-type electric charge accumulating layer 305 accumulates the electric charge generated in the photodiode. A holding electrode 306 maintains the surface of the electric charge accumulating layer in a reverse state. A second transfer electrode 307 receives a pulse for transferring the electric charge accumulated in the electric charge accumulating layer 305. A floating diffusion 308 accumulates the electric charge transferred from the electric charge accumulating layer 305 again and converts the electric charge to a voltage signal. A light shielding film 309 shields the portion other than the photodiode element from light. A P-type well region 310 forms a reverse biased PN junction with the N-type layers such as the electric charge accumulating layer 305 and the floating diffusion 308. A deep P well layer 311 forms a retrograde potential so that the electric charge generated by photoelectric conversion in a deep position inside the silicon is collected in the photodiode element. An N-type overflow drain region 312 serves to discharge the electric charge overflowed from the photodiode to the power source. A first transfer electrode 1101 controls the potential barrier so that the electric charge generated in the photodiode is accumulated in the electric charge accumulating layer 305. An overflow drain control electrode 1102 discharges excess electric charge in the photodiode to a power source when a predetermined potential is applied.
FIGS. 9B to 9G are each an electron potential diagram corresponding to the cross section of FIG. 9A illustrating the electric charge generated in the photodiode until being transferred to the floating diffusion. FIG. 9B is an electron potential diagram before light is entered. FIG. 9C illustrates a state where light is entered and then the photodiode performs photoelectric conversion on the light to generate electric charge which flows into the electric charge accumulating layer. FIG. 9D illustrates a state where a positive voltage is applied to the first transfer electrode 1101 and the electric charge is completely transferred from the photodiode to the electric charge accumulating layer 305. FIG. 9E illustrates a state where a positive voltage is applied to the overflow drain electrode 1102 to discharge the electric charge entered into the photodiode. FIG. 9F illustrates a state where a positive voltage is applied to the second transfer electrode 307 by an operation of an electronic shutter and the electric charge is being transferred from the electric charge accumulating layer 305 to the floating diffusion 308. Finally, FIG. 9G illustrates a state where the generated electric charge has been completely transferred to the floating diffusion 308.
Note that according to the configuration disclosed in aforementioned Patent Document 2, two methods are disclosed as the method of controlling the potential barrier by the first transfer electrode 1101 located between the photoelectric conversion element and the accumulating element. One method is to use a surface channel MOS transistor to always keep the transistor weakly turned ON during the photoelectric conversion period. The other method is to use a buried channel MOS transistor to provide a low potential position at a certain depth from the surface while keeping the transistor strongly turned OFF during the photoelectric conversion period. According to Patent Document 2, the photoelectric conversion element can be reduced to a minimum size required to receive light. This configuration enables an intra-surface synchronized electronic shutter for synchronizing the start time and the end time to accumulate all pixels in the surface.
Alternatively, a document “Dynamics Suppression of Interface-State Dark Current in Buried-Channel CCDs” IEEE Transactions On Electron Devices, VOL. 38, No. 2, February 1991 (hereinafter referred to as Non-Patent Document 1) reveals that an average dark current at the time of transfer can be reduced by shortening a transfer period per transfer in a transfer channel of the buried channel CCD (Charge Coupled Device).
As the method of controlling the potential barrier by the first transfer electrode 1101, consider generating a potential structure as illustrated in FIG. 9B by a method of using a surface channel MOS transistor. In this case, there arises a problem in that dark current increases due to depletion of a substrate surface constituting an MOS transistor of the transfer element. In order to cause electrons to flow from the photoelectric conversion element into the accumulating element, the potential barrier needs to be lowered. In order to do this, the above described MOS transistor needs to be turned ON. At this time, the substrate surface is weakly inverted or strongly inverted, thereby generating electrons which are seen as dark current.
Next, as the method of controlling the potential barrier by the first transfer electrode 1101, consider generating a potential structure as illustrated in FIG. 9B by a method of using a buried channel MOS transistor. In this case, in FIG. 9B, a high channel potential portion appears in a position at a certain depth from the surface. Then, the electric charge generated by photoelectric conversion in the photodiode element is shielded by the potential barrier generated by the MOS transistor. Therefore, some electric charge is accumulated in the photodiode and the overflowed electric charge is sequentially sent to the accumulating element as a signal charge. While electrons are being sent to the accumulating element, a conductive carrier opposite to the signal charge is accumulated in the substrate surface. Thereby, dark current is suppressed from occurring.
However, if photoelectric conversion is performed for a long time in a state where the electric charge is accumulated in the photodiode, the internal electric field of the photodiode continues to be weak during the photoelectric conversion period, thereby increasing the probability that the generated electric charge is escaped from the photodiode to adjacent pixels or the substrate without staying therein. Thus, an increase in the probability that the generated electric charge is not accumulated in the accumulating element but is escaped to other places causes cross talk or color mixture or photo response non-uniformity (PRNU) to increase. Here, cross talk or color mixture means that in a color photoelectric conversion apparatus, the electric charge generated by light entered into a pixel is escaped to adjacent pixels and exhibits a color different from the original color. Photo response non-uniformity (PRNU) means a degree of unevenness of an image formed due to a variation in signal intensity for each pixel caused by a variation in sensitivity of the pixel itself or a variation in gain of the reading system with respect to the same incident light intensity. Both cross talk or color mixture and PRNU are undesirable for the imaging apparatus.
An object of the present invention is to solve the aforementioned problems and to provide a solid-state imaging apparatus, a driving method of the same and an imaging system capable of reducing cross talk or color mixture or photo response non-uniformity (PRNU) by suppressing dark current from occurring.